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Driving Grado Headphones
The Grado headphone's brutally low impedance (about 30 ohms) can follow the same procedure as the Sennheiser headphones. Where the 300-ohm loads required a 900-ohm plate resistor values, the 30 ohms load requires a 90-ohm load. But with a 90-ohm plate resistor and a 5687 input tube, the final gain is only 0.2, hardily enough. Increasing the gain can be accomplished by adding an additional gain stage or more output tubes, but a better choice might be to use an output transformer with this circuit. Still, keeping the output stage transformerless is worth pursuing. Increasing the number of parallel output triodes to seven would demand a sevenfold increase of the plate resistor's nominal value of 30 ohms (210 ohms). This value of plate resistor yields a final gain of 0.7, which is much more usable. The added bonus is the corresponding decrease in output impedance down to 18 ohms. Finally, 5687s are so cheap and reliable that few would balk at the price of a re-tubing every 5 years.
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